A. Apparent Motions of the Sun, the Stars,
and the Celestial Sphere
Days are caused by the Earth's spin. The Sun and most stars rise in the east and set in
the west.
Celestial Sphere = an imaginary sphere surrounding the Earth. The stars are fixed
to this sphere, and the Sun, Moon, and planets move across it. It has poles and an equator
(extensions of the Earth's poles and equator).
The celestial sphere has a coordinate system like latitude and longitude.
- Declination (Dec.) measures angle from the celestial equator (like
latitude). It
runs from +90o (north pole) to -90o (south pole).
- Right Ascension (RA) measures angles perpendicular to dec (like longitude). RA
divides the sky into 24 hours instead of 360 degrees.
Your latitude on the Earth determines which stars rise and set, which stay up all of the
time, and which never set.
Years are caused by the Earth's orbit around the Sun. The Sun appears to move west to
east against the background stars over a year.
Solar Day = time from noon to noon (or midnight to midnight) = 24 hours.
Sidereal Day = time required for the Earth to rotate 360 degrees, so stars appear
in the same place in the sky = 23 hours, 56 minutes.
The Earth's tilt (23.5o) causes seasons. When the northern hemisphere
is pointed toward the Sun, we have northern hemisphere spring and summer (and southern
hemisphere fall and winter). The opposite is true when the northern hemisphere is pointed
away from Sun.
The ecliptic = the path the Sun follows across the celestial sphere, west to
east. It is tilted with respect to the celestial equator.
- When the Sun is furthest north, northern summer solstice (June 21).
- When the Sun is furthest south, northern winter solstice (December 21).
- When the Sun crosses the celestial equator, vernal (March 21) and autumnal
(September 22) equinoxes.
The point on the ecliptic where the Sun crosses the celestial equator going north
(vernal equinox) marks 0 hours R.A.
On the short term, stars are fixed to the celestial sphere.
On the longer term, the poles `precess' (26000 year period) due to the pull of the Moon
and Sun on the Earth, and the stars move with respect to each other (thousands to millions
of years). B. Apparent Motions and Phases of the Moon
The Moon rises in the East and sets in the West daily.
The Moon moves W-> E with respect to the stars (27.3 days = 1
sidereal month).
The Moon moves with respect to the Sun.
Lunar phases depend on the positions of the Earth, Moon, and Sun. It takes 29.5 days to
go through full cycle of phases (1 synodic month). The phase are called new (no
visible disk), 1st quarter (half moon visible), full (full disk visible), 3rd
(or last) quarter (half moon visible).
Full moon: rises at sunset.
New moon: rises at sunrise.
The Moon rotate once each month. This keeps the same face towards the Earth. C.
Eclipses
C.1 Solar Eclipse: Moon passes between the Earth and the Sun
Moon's shadow cast on the Earth.
Total eclipse occurs in when the Earth's surface is in the umbra (complete
shadow).
Partial and annular eclipses occur where the Earth's surface is in the penumbra
(partial shadow).
In an annular eclipse, the umbra doesn't reach the Earth.
Phase: New Moon. C.2 Lunar Eclipse:
Earth passes between the Sun and the Moon
Earth cast's a shadow on the Moon.
Earth's umbra can completely cover the Moon.
Red light bent by the Earth's atmosphere still reaches the Moon.
A lunar eclipse is visible from any place on Earth that can see the Moon
(i.e.. night
hemisphere).
Phase: Full Moon.
Note: The Moon's orbit is tilted with respect to the ecliptic (the Sun's path
across the sky), so eclipses don't occur at every full and new Moon. D. The Apparent
Motions of the Planets
Five planets (ancient Greek for ``wanderers'') can be seen without a telescope:
Mercury, Venus,
Mars, Jupiter, and
Saturn. They also move with respect to the stars.
Mercury and Venus stay close to the Sun (28o and 47o;
respectively).
Mars, Jupiter, Saturn, can be up to 180o from the Sun.
All planets usually move W -> E with respect to the stars: prograde motion.
Sometimes planets appear to move backwards: retrograde motion.
All planets stay close to (but not exactly on) the ecliptic.
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